A centrifugal pump may generally include a wear ring assembly for restricting the flow of pumped fluid from a discharge pressure region to a lesser pressure region within the pump. For example, a wear ring assembly may be used to inhibit the flow of fluid that would otherwise occur between a discharge pressure and a suction pressure within a pump.
A wear ring assembly typically has an inner wear ring oriented coaxially with respect to a stationary outer wear ring. The stationary outer wear ring is normally pressed into a pump housing. Variations in radial clearance between the inner wear ring and the outer wear ring result from adding various deviations from ideal dimensions or tolerances within the pump. The inherent limitations of manufacturing processes or materials (e.g., polymers) for manufacturing pumps may lead to deviations from optimum pump tolerances. The inner wear ring does not often have an ideal or entirely concentric rotational relationship with respect to the outer wear ring because of deviations from optimum pump tolerances. Accordingly, during operation of the pump, unwanted frictional contact occurs between the inner and outer wear ring, resulting in heating of the pumped fluid, drag on the drive motor of the pump, and potential contaminants in the pumped fluid from depletion of the surfaces of the wear ring assembly. In addition, if a desired minimal radial clearance between the inner wear ring and the outer wear ring is exceeded because of variations from optimum pump tolerances, the wear ring assembly may provide an inadequate flow barrier between a discharge pressure and a suction pressure of the pump. Thus, a need exists for a pump design or improvement that can compensate for a slightly eccentric relationship or coaxial misalignment between the inner wear ring and the outer wear ring of a wear ring assembly.
In the past, pump designs have featured wear rings pressed onto an outer diameter of an impeller lip. Wear rings pressed onto the outer diameter of an impeller lip typically suffer from two infirmities. First, a wear ring mounted onto the diameter of an impeller tends to be forced off the impeller by hydraulic forces during operation of the pump. The back side of the wear ring experiences discharge pressure with axial forces from fluid that creeps behind the back side. In contrast, the front side of the wear ring experiences suction pressure, which is significantly lower than the discharge pressure. Accordingly, the net hydraulic force acting on the wear ring tries to push it off the impeller toward the inlet of the pump. Even if the net hydraulic force merely slides the wear ring slightly forward toward the pump inlet, a catastrophic failure of the pump may occur. For example, in one failure mode the wear ring jams the rotation of the impeller by contacting stationary casing material which would normally be located axially in front of the wear ring by a clearance dimension.
A second infirmity of the foregoing mounting arrangement, predominately applies in the context of ceramic wear rings. Mounting a ceramic wear ring on the outer diameter of an impeller lip places the wear ring in tension, increasing the chance of failure or breakage of the ceramic wear ring. Ceramic wear rings feature low tensile strength and high compressive strength. Ceramic wear rings can be made radially thicker to compensate for certain tensile forces.
Other pump designs use an elastomer o-ring that engages a groove in a wear ring and an impeller to affix the wear ring to the impeller. The o-ring is composed of a flexible elastomer that has sufficient give to permit assembly. Providing the requisite flexibility of the o-ring typically results in an undesired compromise in structural integrity of the o-ring. Accordingly, the o-ring attachment of the wear ring to the impeller may lack reliability over extended periods of operation. For example, the wear ring may become separated or misaligned with respect to the impeller because the o-ring breaks or yields from a lack of adequate shear or tensile strength during pump operation. Thus, a need exists for a reliable mounting technique for mounting wear rings on an impeller, including ceramic wear rings.